Earlier studies suggested that the cis-trans isomerism of peptide bonds of prolyl residues might be the rate limiting process in the unfolding and refolding of small globular proteins. That this is probably correct has been confirmed by kinetic studies of the refolding of parvalbumins; some of which contain no prolines. It seems also that many slow equilibrations of native proteins between two well-folded states, a common occurrence, may be kinetically controlled by peptide isomerism. This idea is currently being tested.